I've seen this expression in two spacetime dimensions, $$ \langle \bar{\psi}(x) \psi(0) \rangle = \gamma^\mu{\partial_\mu} \langle \phi(x) \phi(0) \rangle $$ The LHS is the fermion propagator, and the expectation on RHS is the scalar propagator. For 2 dimensional case, the scalar propagator is ...

At every point of the 4-D space-time, it's metric, being a symmetric 2-tensor, has $\frac{D(D+1)}{2}=10$ independent components. From this we can subtract four degrees of freedom according to the four coordinate transformations; Hence, we have six independent degrees of freedom at every point. Th...